Method and apparatus for power driving

ABSTRACT

Aspects of the disclosure provide a power circuit to provide electric energy with control and protection for driving a load, such as a light emitting diode (LED) array, and the like. The power circuit includes a converter, a voltage feedback module, a current feedback module and a controller. The converter is configured to receive electric energy from an energy source and to deliver the electric energy for driving the load. The voltage feedback module is configured to generate a first feedback signal based on a voltage of the delivered electric energy. The current feedback module is configured to generate a second feedback signal based on a current of the delivered electric energy. The controller is configured to receive the first feedback signal and the second feedback signal, and to control the converter to receive and deliver the electric energy based on the first feedback signal and the second feedback signal.

INCORPORATION BY REFERENCE

This application is a continuation of U.S. application Ser. No.12/884,255, filed Sep. 17, 2010, which claims the benefit of U.S.Provisional Application No. 61/249,506 “Low Cost LED Voltage and CurrentFeedback Loop Circuit” filed on Oct. 7, 2009, which is incorporatedherein by reference in its entirety.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Light emitting diode (LED) lighting devices provide the advantages oflow power consumption and long service life. Thus, LED lighting devicesmay be used as general lighting equipment in the near future to replace,for example, fluorescent lamps, bulbs, and the like.

SUMMARY

Aspects of the disclosure provide a power circuit to provide electricenergy with control and protection for driving a load, such as a lightemitting diode (LED) array, and the like. The power circuit includes aconverter, a voltage feedback module, a current feedback module and acontroller. The converter is configured to receive electric energy froman energy source and to deliver the electric energy for driving theload. The voltage feedback module is configured to generate a firstfeedback signal based on a voltage of the delivered electric energy. Thecurrent feedback module is configured to generate a second feedbacksignal based on a current of the delivered electric energy. Thecontroller is configured to receive the first feedback signal and thesecond feedback signal, and to control the converter to receive anddeliver the electric energy based on the first feedback signal and thesecond feedback signal.

In an embodiment, the converter includes a transformer having a primarywinding on a receiving path and a secondary winding on a deliveringpath, and a switch on the receiving path. The switch is configured toswitch on the receiving path to receive and store the electric energyand to switch off the receiving path to allow the delivering path todeliver the stored electric energy to the load. The controller isconfigured to control a turn-on time of the switch based on a dominantfeedback signal of the first feedback signal and the second feedbacksignal.

In an embodiment, the voltage feedback module includes a voltage dividerconfigured to generate the first feedback signal by dividing the voltageon the delivering path. Further, the current feedback module includes asensing module configured to generate the second feed back signal as afunction of a load current. In an example, the sensing module includes aresistor coupled with the load in series. The sensing module senses avoltage drop on the resistor. In addition, in an example, the sensingmodule includes a scaling circuit configured to scale the voltage dropon the resistor.

According to an aspect of the disclosure, the controller is configuredto reduce the turn-on time when the dominant feedback signal is largerthan a limit. In an implementation example, the controller includes afirst diode and a second diode coupled together. The first diode has afirst anode receiving the first feedback signal and a first cathode. Thesecond diode has a second anode receiving the second feedback signal anda second cathode that is coupled with the first cathode to output thedominant feedback signal.

Aspects of the disclosure provide an LED lighting device that includesthe power circuit and an LED array. The power circuit provides electricenergy to the LED array. The LED array emits light in response to thereceived electric energy.

Aspects of the disclosure provide a method for driving an LED array. Themethod includes generating a first feedback signal based on a voltage ofelectric energy delivered from a converter for driving the LED array,generating a second feedback signal based on a current of the electricenergy delivered to the LED array, and controlling the converter toreceive and deliver the electric energy based on the first feedbacksignal and the second feedback signal.

To control the converter to receive and deliver the electric energybased on the first feedback signal and the second feedback signal, themethod includes controlling a turn-on time of a switch that couples areceiving path to receive electric energy and to store the receivedelectric energy in a transformer. The transformer has a primary windingon the receiving path and a secondary winding on a delivering path. Inan example, the method includes selecting a dominant feedback signal ofthe first feedback signal and the second feedback signal, andcontrolling the turn-on time of the switch based on the dominantfeedback signal.

To generate the first feedback signal, the method includes dividing avoltage on the delivering path. To generate the second feedback signal,the method includes sensing a voltage drop on a resistor that is coupledwith the LED array in series.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of this disclosure that are proposed as exampleswill be described in detail with reference to the following figures,wherein like numerals reference like elements, and wherein:

FIG. 1A shows a block diagram of a power system example 100A accordingto an embodiment of the disclosure;

FIG. 1B shows a block diagram of a comparison power system example 100B;

FIG. 2 shows a block diagram of a power system example 200 according toan embodiment of the disclosure;

FIG. 3 shows a block diagram of another power system example 300according to an embodiment of the disclosure;

FIG. 4A shows a plot 400A tracing electrical parameters during a coldstart-up of the power system example 200 according to an embodiment ofthe disclosure;

FIG. 4B shows a plot 400B tracing a voltage in the power system example200 according to an embodiment of the disclosure;

FIG. 4C shows a plot 400C tracing electrical parameters during a warmstart-up of the power system example 200 according to an embodiment ofthe disclosure;

FIG. 5 shows a flow chart outlining a process example 500 for the powercircuit 101A to drive the load 120A according to an embodiment of thedisclosure; and

FIG. 6 shows a flow chart outlining a process example 600 for thecontroller 240 to generate pulses according to an embodiment of thedisclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1A shows a block diagram of a power system example 100A accordingto an embodiment of the disclosure. The power system 100A includes apower circuit 101A and a load 120A. The power circuit 101A receiveselectric energy from an energy source, and delivers the electric energywith regulated voltage and regulated current to the load 120A. In anembodiment, the power circuit 101A includes a converter 110A, a voltagefeedback module 150A, a current feedback module 160A, and a controller140A. These elements are coupled together as shown in FIG. 1A.

The converter 110A receives input of electric energy from any suitableenergy source, and delivers the electric energy under the control of thecontroller 140A to achieve the regulated voltage and the regulatedcurrent. In an embodiment, the converter 110A includes a transformer111A, a switch 113A, a rectifier diode 112A, and a capacitor 114A. Theseelements are coupled together as shown in FIG. 1A.

Specifically, the transformer 111A includes a primary winding 111A-P,and a secondary winding 111A-S. The primary winding 111A-P and theswitch 113A are coupled in series, for example, to form a receiving pathto receive the electric energy from an energy source. The energy sourcecan be any suitable energy supply, such as an AC voltage supply, a DCvoltage supply and the like. The power circuit 101A can includecomponents to suitably process the input from the energy source. In anexample, the power circuit 101A includes a bridge rectifier (not shown)to rectify the input from an AC voltage supply. Further, the powercircuit 101A includes a filter circuit (not shown) to reduce noisescoming from the AC voltage supply. In addition, the power circuit 101Acan include a monitor circuit (not shown) to monitor the input, such asan input voltage Vin, from the AC voltage supply.

The switch 113A controls the receiving path to receive electric energyfrom the energy source. In an example, when the switch 113A is switchedon, the receiving path is coupled to the energy source to receive theelectric energy; and when the switch 113 is switched off, the receivingpath is decoupled from the energy source.

The secondary winding 111A-S, the rectifier diode 112A, and thecapacitor 114A can be coupled together to form a delivering path todeliver electric energy to a load, such as the load 120A. The powercircuit 101A can be suitably configured into an isolated configurationor a non-isolated configuration. In the FIG. 1A example, the powercircuit 101A is configured into a non-isolated configuration.Specifically, a terminal of the receiving path is connected to ground(i.e., a terminal of the switch 113A), and a terminal of the deliveringpath is also connected to the same ground (i.e., a terminal of thesecondary winding 111A-S). Thus, in this example, the receiving path andthe delivering path are not isolated.

Further, the transformer 111A, the rectifier diode 112A, and the switch113A are configured to enable a flyback transformer operation. Morespecifically, when the switch 113A is switched on, the receiving path iscoupled to the energy source to receive electric energy, and a switchcurrent Ip flows through the primary winding 111A-P of the transformer111A, and the switch 113A. The polarity of the transformer 111A and thedirection of the rectifier diode 112A are arranged, such that there isno current in the secondary winding 111A-S of the transformer 111A whenthe switch 113A is switched on. Thus, the received electric energy isstored in the transformer 111A.

When the switch 113A is switched off, the receiving path is decoupledfrom the energy source, and the switch current Ip becomes zero. Thepolarity of the transformer 111A and the direction of the rectifierdiode 112A enable the delivering path to deliver the stored electricenergy to the load 120A.

According to an embodiment of the disclosure, the switch 113A isrepetitively switched on and off. Thus, the receiving path repetitivelyreceives the electric energy and stores the electric energy in thetransformer 111A. Then, the delivering path repetitively delivers thestored electric energy to the load 120A.

According to an embodiment of the disclosure, the switch 113A issuitably controlled by a relatively high frequency pulse signal. In anembodiment, the AC voltage supply has a frequency of 50 Hz. Thecontroller 140A provides a pulse signal having a much higher frequencythan 50 Hz, for example, in the order of KHz or MHz, to switch on andswitch off the switch 113A. Each time, the controller 140A switches onand switches off the switch 113A, the switch current Ip has a spike. Thepeak value of the spike is a function of the AC voltage supply duringthe switch-on time. Thus, an average of the switch current Ip hassubstantially the same phase as the voltage of the AC voltage supply.Thus, using the relatively high frequency pulse signal, the powercircuit 101A enables power factor correction (PFC).

Additionally, in an embodiment, a pulse width of the relatively highfrequency pulse signal is adjustable. The pulse width determines aturn-on time of the switch 113A. The turn-on time further determines theamount of electric energy received, stored and delivered by theconverter 110A in a switching cycle.

The voltage feedback module 150A uses any suitable technique to generatea first feedback signal as a function of a delivered voltage from theconverter 110A. The current feedback module 160A uses any suitabletechnique to generate a second feedback signal as a function of adelivered current from the converter 110A. In an embodiment, both thefirst feedback signal and the second feedback signal are voltagesignals. In an example, the first feedback signal is generated by avoltage divider that divides the delivered voltage according to apredetermined ratio. The second feedback signal is generated as avoltage drop on a resistor that the delivered current flows through.

The controller 140A generates pulses based on the first feedback signaland the second feedback signal to control the switch 113A. Thus, theconverter 110A, the voltage feedback module 150A, the current feedbackmodule 160A and the controller 140A form a feedback control loop toprovide energy control and circuit protection at various operationstates for the power circuit 101A and the load 120A.

In an example, the load 120A includes a light emitting diode (LED) array(not shown) for emitting light in response to the delivered electricenergy. The power circuit 101A provides electric control and circuitprotection to the LED array at a no-load state, a current-up state, aheat-up state, and a steady state, for example.

In the no-load state, the LED array fails to be the load or appears tobe no-load to the power circuit 101A. In an example, the LED array failsto be electrically coupled to the delivering path due to, for example, abroken wire. In another example, the LED array requires a forwardvoltage to enable conducting current. When the voltage on the deliveringpath is smaller than the forward voltage, the LED array does not conductcurrent, and the LED array appears as no-load to the delivering path.

In the no-load state, the voltage on the delivering path rises rapidly,and can reach dangerous levels if suitable protection technique is notapplied. According to an embodiment of the disclosure, in the no-loadstate, the first feedback signal is dominant, and the controller 140Agenerates pulses based on the first feedback signal. When the firstfeedback signal, which is generated as a function of the voltage on thedelivering path, is larger than a threshold, the controller 140Atemperately stops generating pulses, and thus the voltage on thedelivering path stops rising.

In another embodiment, when the first feedback signal is larger than athreshold, the controller 140A adjusts the pulse width to reduce theturn-on time of the switch 113A, and thus the voltage on the deliveringpath rises relatively slowly.

When the voltage on the delivering path is larger than the forwardvoltage, the power system 100A enters the current-up state. In thecurrent-up state, the LED array starts conducting current, and startsemitting light. Generally, the LED array has a relatively slow response,thus the conducted current rises slowly. Thus, in the current-up state,the first feedback voltage is still dominant, and the controller 140Agenerates pulses based on the first feedback signal.

According to an embodiment of the disclosure, when the first feedbacksignal, which is generated as a function of the voltage on thedelivering path, is larger than a threshold, the controller 140A reducesthe pulse width to reduce the turn-on time of the switch 113A, and thusthe voltage on the delivering path does not rise rapidly in thecurrent-up state. In another embodiment, the controller 140A temperatelystops generating pulses.

When the conducted current of the LED array reaches a specific level,such as a level that causes the second feedback signal to be larger thanthe first feedback signal, the power system 100A enters the heat-upstate. In the heat-up state, the second feedback signal is dominant. Thecontroller 140A generates pulses based on the second feedback signal tomaintain a relatively stable current flowing through the LED array, andoffset variations due to temperature changes. Generally, the lightintensity is a function of the conducted current by the LED array. Whenthe relatively stable current flows through the LED array, the lightemitted by the LED array has a relatively stable light intensity.

In an example, when the second feedback signal, which is generated as afunction of the delivered current to the LED array, is larger than anupper bound, the controller 140A reduces the pulse width to reduce theturn-on time of the switch 113A, and thus reduces the electric energydelivered to the LED array. On the other hand, when the second feedbacksignal is smaller than a lower bound, the controller 140A increases thepulse width to increase the turn-on time of the switch 113A, and thusincreases the electric energy delivered to the LED array.

Additionally, in the heat-up state, as the LED array conducts currentand emits light, the temperature of the LED array rises, and causeselectrical properties, such as the forward voltage, I-V characteristic,and the like, to change. The controller 140A adjusts the pulse width tooffset the temperature-induced variations, and thus keeps the emittedlight intensity to be relatively stable.

At some point, the temperature stops rising and stays in a relativelystable range, then the power system 100A enters the steady state. In thesteady state, the second feedback signal is dominant, and the controller140A generates pulses based on the second feedback signal to maintainthe relatively stable current flowing through the LED array. Forexample, when the second feedback signal, which is generated as afunction of the delivered current to the LED array, is larger than anupper bound, the controller 140A reduces the pulse width, and thusreduces the electric energy delivered to the LED array. On the otherhand, when the second feedback signal is smaller than a lower bound, thecontroller 140A increases the pulse width, and thus increases theelectric energy delivered to the LED array.

It is noted that the controller 140A can also generate pulses based onother suitable signals, such as a signal CLOCK that is indicative of aswitching clock, a signal Vin that is indicative of an input voltage, asignal Ip that is indicative of the current flowing through the switch113A, and the like.

FIG. 1B shows a comparison power system 100B. The comparison powersystem 100B utilizes certain components that are identical or equivalentto those used in the power system 100A; the description of thesecomponents has been provided above and will be omitted here for claritypurposes. However, the comparison power system 100B does not include avoltage feedback module to provide the first feedback signal to thecontroller 140B. The controller 140B controls the switch 113B based onthe feedback signal, which is generated as a function of the currentflowing through the load 120B. The comparison power system 100B maysuffer damages due to excess voltage on the delivering path. In anexample, the load 120B is not electrically coupled to the deliveringpath. Due to the reason that the feedback control loop is broken, thevoltage on the delivering path continually rises, and can cause damages.

In another example, the load 120B, such as an LED array, is electricallycoupled to the delivering path. However, the feedback control loop isrelatively slow due to, for example, a slow reaction of the LED array, adelay caused by pulse-based PFC, and the like. Thus, before the feedbackcontrol loop reacts effectively, the voltage on the delivering pathrises rapidly and reaches dangerous levels.

It is noted that various techniques can be used to implement the voltagefeedback module 150A, the current feedback module 160A, and thecontroller 140A.

FIG. 2 shows a block diagram of a power system example 200 according toan embodiment of the disclosure. The power system 200 includes a powercircuit 201 that drives an LED array 220. The power system 200 utilizescertain components that are identical or equivalent to those used in thepower system 100A; the description of these components has been providedabove and will be omitted here for clarity purposes.

In the FIG. 2 example, the converter 210 receives electric energy froman AC voltage supply. The converter 210 includes a bridge rectifier 215that rectifies the input from the AC voltage supply.

The voltage feedback module 250 includes two resistors R1 and R2 thatare coupled in series to form a voltage divider to generate the firstfeedback signal as a function of the voltage on the delivering path. Theresistance ratio of R1 and R2 determines the ratio of the voltagedivider.

The current feedback module 260 includes a sensing resistor R3 that iscoupled with the LED array 220 in series to conduct the same current asthe LED array 220. The current feedback module 260 generates the secondfeedback signal as a voltage drop on the sensing resistor R3, which is afunction of the conducted current of the LED array 220. Further, thecurrent feedback module 260 includes a capacitor C1 and a resistor R4that form a filter to reduce high frequency components in the secondfeedback signal.

The controller 240 controls the switch 213 based on the first feedbacksignal and the second feedback signal. In an embodiment, the controller240 is implemented using a circuit 245 and a combiner circuit 248. Thecircuit 245 is an existing control circuit, such as an existingintegrated circuit (IC) chip, an existing intellectual property (IP)block, and the like, that generates a pulse signal based on a feedbacksignal. In an example, the circuit 245 includes a feedback pin toreceive the feedback signal, and a switch control pin to provide thepulse signal to the switch 213. The combiner circuit 248 combines thefirst feedback signal with the second feedback signal, and provides thecombined feedback signal to the feedback pin of the circuit 245.

In an embodiment, the combiner circuit 248 is implemented using athree-pin diode package 248 to provide a relatively low cost solution tocombine the first feedback signal and the second feedback signal. Thethree-pin diode package 248 includes a first diode D1 having a firstanode and a first cathode, and a second diode D2 having a second anodeand a second cathode. The three-pin diode package 248 includes a firstanode pin for the first anode, a second anode pin for the second anode,and a cathode pin coupled with the first cathode and the second cathode.In an example, the first anode pin receives the first feedback signal,the second anode pin receives the second feedback signal, and the sharedcathode pin provides the combined feedback signal to the circuit 245.

In an embodiment, the circuit 245 operates based on a voltage signalreceived on the feedback pin. In an example, when the voltage signalreceived at the feedback pin is larger than 2.5V, the circuit 245reduces the pulse width of the generated pulses. In another example, thecircuit 245 temperately stops generating pulses.

In the FIG. 2 example, the voltage signal received on the feedback pinis determined by the dominant one of the first feedback signal and thesecond feedback signal. For example, when the first feedback signal islarger than the second feedback signal, the voltage signal on thefeedback pin is determined by the first feedback signal; and when thefirst feedback signal is smaller than the second feedback signal, thevoltage signal on the feedback pin is determined by the second feedbacksignal.

In an example, the ratio of the voltage divider is 0.22. When thevoltage on the delivering path is larger than 13V, the first feedbacksignal is larger than 2.8V. Then, when the first diode D1 has a forwardvoltage of 0.3V, the voltage signal received on the feedback pin islarger than 2.5V.

In another example, the current flowing through the LED array is largerthan 350 mA, and the sensing resistor R3 has a resistance of 8.2Ω, thusthe second feedback signal is larger than 2.8V. Then, when the seconddiode D2 also has a forward voltage of 0.3V, the voltage signal on thefeedback pin is larger than 2.5V.

During operation, for example, when an external switch (not shown) isswitched on, the AC supply is coupled to the power circuit 201, and thepower system 200 enters the no-load state. In the no-load state, thecircuit 245 generates pulses to have an initial pulse width. The pulsescontrol the switch 213 to receive and deliver electric energy to the LEDarray 220. However, the LED array 220 does not conduct current beforethe forward voltage requirement is satisfied, and thus the voltage onthe delivering path rises rapidly

In the no-load state, the first feedback signal is dominant. In anexample, when the first feedback signal is smaller than 2.8V, thefeedback pin is smaller than 2.5V, and the circuit 245 maintains orincreases the pulse width. When the first feedback signal is larger than2.8V, the feedback pin is larger than 2.5V, and the circuit 245 reducesthe pulse width.

When the voltage on the delivering path satisfies the forward voltage ofthe LED array 220, the power system 200 enters the current-up state. Inthe current-up state, the LED array starts conducting current, andstarts emitting light. Generally, the LED array 220 has a relativelyslow response, thus the conducted current rises slowly. In thecurrent-up state, the first feedback signal is still dominant. Thecircuit 245 controls the pulse generation based on the first feedbacksignal to prevent the voltage on the delivering path to rise todangerous level.

When the conducted current reaches a specific value that the secondfeedback signal is larger than the first feedback signal, the secondfeedback signal is dominant, and the power system 200 enters the heat-upstate. In the heat-up state, the circuit 245 controls the pulsegeneration based on the second feedback signal to maintain substantiallystable current delivered to the LED array 220. In addition, in theheat-up state, the temperature of the LED array 220 rises. The risingtemperature causes various electrical properties of the LED array 220 tochange. For example, when temperature rises, the forward voltage of theLED array 220 drops, and the LED array 220 tends to conduct more currentwith the same delivered voltage. As the conducted current rises, thesecond feedback signal rises. In an example, when the second feedbacksignal is larger than 2.8V, the voltage on the feedback pin of thecircuit 245 is larger than 2.5V, and the circuit 245 generates pulseswith reduced pulse width. The pulses are provided to the switch 213 toreduce the turn-on time, and to reduce the electric energy delivered tothe LED array 220. The reduced electric energy offsets the forwardvoltage drop of the LED array 220.

At certain point, the temperature becomes relatively stable, and thepower system 200 enters the steady state. In the steady state, theforward voltage of the LED array 220 also drops to a relatively stablevalue, and the second feedback signal is dominant. Then, the circuit 245controls the pulse generation based on the second feedback signal tomaintain the relatively stable current delivered to the LED array 220.

FIG. 3 shows another block diagram of a power system example 300according to an embodiment of the disclosure. The power system 300utilizes certain components that are identical or equivalent to thoseused in the power system 200; the description of these components hasbeen provided above and will be omitted here for clarity purposes.

In the FIG. 3 example, the current feedback module 360 includes anoperational amplifier OA and two resistors R5 and R6 coupled together toform a scaling module. The ratio of the scaling module is determined bya ratio of the two resistors R5 and R6. In this example, a relativelysmall R3 can be used to reduce power consumption on the sensing resistorR3.

FIG. 4A shows a plot 400A tracing electrical parameters during a coldstart-up of the power system 200 according to an embodiment of thedisclosure. The cold start-up refers to the LED array having arelatively low temperature, such as a room temperature, at the start-up.The plot 400A includes a voltage curve 410A corresponding to the voltageon the delivering path, and a current curve 420A corresponding to thecurrent flowing through the LED array 220.

The voltage curve 410A and the current curve 420A traces the electricalparameters in the no-load state, the current-up state, and a part of theheat-up state for the power system 200.

In the no-load state, the circuit 245 generates pulses of an initialpulse width to control the switch 213. The voltage on the deliveringpath rises rapidly. However, before the voltage on the delivering pathreaches the forward voltage of the LED array 220, the LED array 220 doesnot conduct current. Thus the LED array 220 appears as no-load to thepower circuit 201.

In the no-load state, the first feedback signal is larger than thesecond feedback signal. However, before the first feedback signalreaches 2.8V, for example, the circuit 245 continues generating thepulses with the initial pulse width. When the voltage on the deliveringpath causes the first feedback signal larger than 2.8V, the circuit 245generates pulses of reduced pulse width to slow down the voltage on thedelivering path. Further, when the voltage on the delivering pathreaches the forward voltage of the LED array 220, the LED array 220starts to conduct current, and the power system 200 enters thecurrent-up state.

In the current-up state, the conducted current by the LED array 220rises. However, before the conducted current reaches certain level, thefirst feedback signal is larger than the second feedback signal. Thecircuit 245 continues generating pulses based on the first feedbacksignal to constrain the voltage on the delivering path. When theconducted current reaches certain level, the second feedback signal islarger than the first feedback signal. Then, the power system 200 entersthe heat-up state.

In the heat-up state, the temperature of the LED array rises, and causeschanges in the electrical parameters of the LED array 220. For example,the forward voltage starts to drop, and causes possible current rise. Inthe heat-up state, the second feedback signal is larger than the firstfeedback signal, thus the circuit 245 generates the pulses based on thesecond feedback signal to maintain relatively stable current flowingthrough the LED array 220. It is noted that due to the forward voltagedrop of the LED array 220, and the effort by the power circuit 201 tomaintain the relatively stable current, the voltage on the deliveringpath drops.

FIG. 4B shows a curve 410B tracing the voltage on the delivering pathover the heat-up state and the steady state. In the heat-up state, thevoltage on the delivering path drops. In the steady state, thetemperature is stable, the electrical properties of the LED array 220are also stable, and the voltage on the delivering path is also stable.

FIG. 4C shows a plot 400C tracing electrical parameters during a warmstart-up of the power system 200 according to an embodiment of thedisclosure. The curves in the plot 400C have certain properties that areidentical or equivalent to those in the plot 400A; the description ofthese properties has been provided above and will be omitted here forclarity purposes.

The warm start-up refers to the LED array 220 having a relatively hightemperature at the start-up. In an example, the power system 200 startsup before the LED array 220 cools down to the room temperature fromprevious operation. Due to the relatively high temperature, the LEDarray 220 has a relatively low forward voltage. Thus, when the voltageon the delivering path reaches the specific value, such as correspondingto the forward voltage at the relatively low temperature, the currentflowing through the LED array 220 overshoots, as shown by 430C, due tothe reason that the LED current based feedback reacts relatively slow.In an embodiment, the power circuit 201 is suitably designed to allow,for example, about 10% overshoots.

FIG. 5 shows a flow chart outlining a process example 500 for the powercircuit 101A to drive the load 120A according to an embodiment of thedisclosure. The process starts at S501, and proceeds to S510.

At S510, the first feedback module 150A generates a first feedbacksignal as a function of the voltage on the delivering path that deliverselectric energy to the load 120A.

At S520, the second feedback module 160A generates a second feedbacksignal as a function of the current flowing through the load 120A.

At S530, the controller 140A generates pulses based on the firstfeedback signal and the second feedback signal. The pulses are used tocontrol the switch 113 to receive and deliver the electric energy to theload 120A. Then, the process proceeds to S599 and terminates.

FIG. 6 shows a flow chart outlining a process example 600 for thecontroller 240 to generate pulses according to an embodiment of thedisclosure. The process starts at S601 when the external switch of thepower system 200 is switched on, and proceeds to S610.

At S610, the controller 240 initializes a pulse width.

At S620, the controller 240 generates pulses having the pulse width. Thepulses are used to control the converter 210 to receive and deliverelectric energy to the LED array 220.

At S630, the controller 240 receives the first feedback signal and thesecond feedback signal. The first feedback signal is generated as afunction of the voltage on the delivering path, and the second feedbacksignal is generated as a function of the current flowing through the LEDarray 220. In an embodiment, the first feedback signal and the secondfeedback signal are both voltage signals and are suitably scaled.

At S640, the controller 240 selects the dominant one of the firstfeedback signal and the second feedback signal. In an embodiment, thecontroller 240 includes two diodes D1 and D2, such as using the 3-pindiode package 248, to select the larger feedback signal.

At S650, the controller 240 determines whether the selected feedbacksignal is larger than a limit. When the selected feedback signal islarger than the limit, the process proceeds to S660; otherwise, theprocess proceeds to S670.

In S660, the controller 240 reduces the pulse width. Then, the processreturns to S620 to generate pulses having the reduced pulse width.

In S670, the controller 240 maintains or increases the pulse width.Then, the process returns to S620 to generate pulses having themaintained or increased pulse width.

The process 600 stops when the external switch is switched off.

It is noted that the process 600 can be suitably changed with differentimplementation of the controller 240. In an example, instead of usingthe existing circuit 245 and the three-pin diode package 248, a new ICis designed to generate pulses based on the first feedback signal andthe second feedback signal. The new IC can include suitable componentsto perform similar or equivalent functions as the circuit 245 and thethree-pin diode package 248, but with different process.

While the invention has been described in conjunction with the specificembodiments thereof that are proposed as examples, it is evident thatmany alternatives, modifications, and variations will be apparent tothose skilled in the art. Accordingly, embodiments of the invention asset forth herein are intended to be illustrative, not limiting. Thereare changes that may be made without departing from the scope of theinvention.

What is claimed is:
 1. A power circuit, comprising: a converterconfigured to receive electric energy from an energy source, and todeliver the electric energy for driving a load; a voltage feedbackmodule configured to continually generate a first feedback signal bydividing a voltage of the delivered electric energy; a current feedbackmodule configured to generate a second feedback signal based on acurrent of the delivered electric energy, the current feedback moduleincluding a resistor and a filter coupled to the resistor, the filterconfigured to reduce high frequency components of the second feedbacksignal; a combining unit comprising at least one diode, the combiningunit configured to take as inputs the first feedback signal and thesecond feedback signal, and to output a feedback signal; and acontroller configured to receive the feedback signal from the combiningunit and to control the converter to receive and deliver the electricenergy based on the feedback signal.
 2. The power circuit of claim 1,wherein at least one of the at least one diode is reverse biased basedon the first feedback signal or the second feedback signal.
 3. The powercircuit of claim 1, wherein the at least one diode has an anode and acathode, the anode receiving the first feedback signal or the secondfeedback signal.
 4. The power circuit of claim 1, wherein the at leastone diode is a first diode, the combining unit further comprises: thefirst diode having a first anode receiving the first feedback signal anda first cathode; and a second diode having a second anode receiving thesecond feedback signal and a second cathode.
 5. The power circuit ofclaim 4, wherein the second cathode is coupled with the first cathode tooutput the feedback signal corresponding to the first feedback signal orthe second feedback signal.
 6. The power circuit of claim 5, wherein thefeedback signal corresponds to one of the first and second feedbacksignals, the one having a higher positive voltage than the otherfeedback signal at an input of the combining unit.
 7. The power circuitof claim 1, wherein the converter further comprises: a transformerhaving a primary winding on a receiving path and a secondary winding ona delivering path; a switch configured to switch on the receiving pathto receive and store the electric energy and to switch off the receivingpath to allow the delivering path to deliver the stored electric energyto the load; and the controller configured to control a turn-on time ofthe switch based on the feedback signal from the combining unit.
 8. Thepower circuit of claim 7, wherein the voltage feedback module furthercomprises: a voltage divider configured to generate the first feedbacksignal by dividing the voltage of the delivered electric energy on thedelivering path.
 9. The power circuit of claim 1, wherein the currentfeedback module further comprises: a sensing module configured togenerate the second feedback signal as a function of a load current. 10.The power circuit of claim 9, wherein the sensing module comprises: aresistor coupled with the load in series.
 11. A method for driving aload, comprising: receiving, by a converter, electric energy from anenergy source, and delivering the electric energy to drive the load;generating continually, by a voltage feedback module, a first feedbacksignal by dividing a voltage of the delivered electric energy;generating, by a current feedback module, a second feedback signal basedon a current of the delivered electric energy, the current feedbackmodule including a resistor and a filter counted to the resistor;reducing high frequency components of the second feedback signal by thefilter; receiving, by a combining unit having at least one diode, thefirst feedback signal and the second feedback signal as inputs andoutputting a feedback signal; and receiving, by a controller, thefeedback signal from the combining unit and controlling the converter toreceive and deliver the electric energy based on the feedback signal.12. The method of claim 11, further comprising: reverse biasing at leastone of the at least one diode based on the first feedback signal or thesecond feedback signal.
 13. The method of claim 11, further comprising:receiving, by an anode, the first feedback signal or the second feedbacksignal, the at least one diode having the anode and a cathode.
 14. Themethod of claim 11, wherein the at least one diode is a first diode,further comprising: receiving, by a first anode, the first feedbacksignal, the first diode having the first anode and a first cathode; andreceiving, by a second anode, the second feedback signal, a second diodehaving the second anode and a second cathode.
 15. The method of claim14, further comprising: outputting the feedback signal corresponding tothe first feedback signal or the second feedback signal by the secondcathode coupled with the first cathode.
 16. The method of claim 15,further comprising: selecting one of the first and second feedbacksignals as the feedback signal, the one having a higher positive voltagethan the other feedback signal at an input of the combining unit. 17.The method of claim 11, further comprising: primary winding on areceiving path and secondary winding on a delivering path by atransformer; switching on, by a switch, the receiving path to receiveand store the electric energy; switching off, by the switch, thereceiving path to allow the delivering path to deliver the storedelectric energy to the load; and controlling a turn-on time of theswitch based on the feedback signal from the combining unit.
 18. Themethod of claim 17, further comprising: generating, by a voltagedivider, the first feedback signal by dividing the voltage of thedelivered electric energy on the delivering path.
 19. The method ofclaim 11, further comprising: generating, by a sensing module, thesecond feedback signal as a function of a load current.
 20. The methodof claim 19, further comprising: generating, by the sensing modulehaving a resistor coupled with the load in series, the second feedbacksignal.